Thyroglobulin was synthesized, inserted into membranes and glycosylated in a completely heterologous, reconstituted system consisting of a protein synthesizing extract, stripped dog pancreas microsomal membranes and calf thyroid RNA. There is now considerable evidence to support the hypothesis that secreted glycoproteins are synthesized on polysomes which become membrane-bound following synthesis of an N-terminal hydrophobic signal sequence. This signal presequence was found on more than 20 nascent proteins destined for secretion, ovalbumin being the only exception.